1. Field of the Invention
The present invention relates in general to electronic ballast circuits for discharge lamps, and more particularly to an electronic ballast circuit for a discharge lamp which is capable of preventing blackening of the discharge lamp to lengthen the life of the discharge lamp.
2. Description of the Prior Art
In a discharge lamp of the cathode-preheating type such as a fluorescent lamp, an excessively large amount of preheating current may instantaneously be applied to both preheating cathode electrodes or filaments of the discharge lamp at the start of lighting of the discharge lamp, or a high voltage pulse may be applied to the filaments of the discharge lamp before normal preheating, as shown in FIG. 3A. Also, supply power across the discharge lamp must be turned on/off when the discharge lamp is to be replaced with a new one under the condition that it remains at its lighted state as an electronic ballast circuit therefor is operated. In this case, a contact high voltage pulse or spark may be generated across the discharge lamp, as shown in FIG. 4A. The high voltage pulse causes a thermion emitting material coated on the filaments such as a barium oxide to be broken away from the filaments, being evaporated or damaged. The thermion emitting material broken-away from the filaments causes a fluorescent material applied in the discharge lamp to be transformed resulting in blackening of the discharge lamp. The life of the discharge lamp is shortened due to the blackening. Moreover, the high voltage pulse exerts a bad influence on components in a circuit for lighting the discharge lamp, namely, damages the circuit components.
Referring to FIG. 1, there is shown a circuit diagram of a conventional electronic ballast circuit for a discharge lamp. In operation, upon application of a commercial alternating current (AC) power to a well-known rectifying circuit, a rectified direct current (DC) voltage from the rectifying circuit is applied across transistors Q1 and Q2 in different polarities. At this time, a high frequency current is generated from the transistors Q1 and Q2 and then flows to a LC series resonance circuit which is comprised of a coil L1 and condensers C1-C3. The high frequency current through the LC series resonance circuit is instantaneously applied by an excessively large amount to filaments H1 and H2 of the discharge lamp, as shown in FIG. 3A, resulting from a voltage multiplied by a Q value of the LC series resonance circuit. The instantaneously large amount of current results in generation of a very high pulse voltage multiplied by the Q value of the LC series resonance circuit across the condenser C1. As mentioned above, the filaments of the discharge lamp are subjected to the damage resulting from the high voltage pulse.
A non-sinusoidal wave or distorted wave current flows through the discharge lamp even upon application of a sinusoidal wave voltage across the discharge lamp because the discharge lamp has a negative resistance according to its natural characteristic. For this reason, the current flowing through the discharge lamp is high in crest factor even in the normal lighting of the discharge lamp, thereby causing the thermion emitting material coated on the filaments to be broken away from the filaments. The thermion emitting material broken-away from the filaments causes the fluorescent material applied in the discharge lamp to be transformed resulting in the blackening of the discharge lamp. As a result, the life of the discharge lamp is shortened due to the blackening.